1. Field of The Invention
This invention relates to a head travelling mechanism for a floppy disk drive, and more particularly to an improvement of a head travelling mechanism for a floppy disk drive which moves a carriage carrying magnetic heads by a belt through a stepping motor drive.
2. Description of The Prior Art
A floppy disk has been well known which includes a substrate of flexible plastic sheet coated with magnetic layers and is increasingly used over a broad range of applications for computers as data recording media in recent years.
In order to perform requested READ/WRITE actions from and to this kind of floppy disk a floppy disk drive is known which provides rotation of the floppy disk and movement of the magnetic heads to the requested track to perform READY/WRITE actions.
In ordinary cases the floppy disk drive comprises a DC servo-motor which rotates the floppy disk and a carriage to move the magnetic heads thereon to the requested track. A stepping motor is used to drive the carriage.
The driving force of the stepping motor is transmitted through a pulley mounted on its shaft and a feeding belt, an .alpha. belt for example, which is wound around the pulley. This provides a belt feeding mechanism in a limited space with excellent feeding accuracy. Particularly, since the .alpha. belt means is composed in such a way that the feeding belt is fixed to the carriage at its both ends and one run around the pulley and the stepping motor can be arranged in the vicinity of the carriage, as indicated in TOKUKAISHO 49-13140, the .alpha. belt means is extremely preferable for the small-sized floppy disk drive using the micro-floppy disk in the recent years.
In the recent years, however, the data storing density is remarkably increasing in the floppy disk drive and there is such a tendency that the distance between the tracks is getting finer. Thus, it has been requested that the heads must be travelled to the desired position with high accuracy.
Accordingly, in the belt feeding mechanism of the carriage using the stepping motor the minute feeding error in the carriage, which has not become a big problem in the prior art, could be a factor in off-track positioning of the heads, one of the problematical errors. Also of much concerned is a tilted shaft of the stepping motor when it is in the driving state.
In ordinary cases, in the stepping motor constructed with a plurality of electrode teeth of the stator cores determined by their magnetic poles in which the N pole and S pole are mixed in the circumferential direction, the motor has less inclination of its shaft and the rotor is always attracted to all directions, since the rotor mounted on the motor shaft has equal magnetic attraction in the circumferential direction. Thus, the motor shaft is received by bearings in a floating state, which is favorable as far as the tilting error on the motor shaft is concerned.
In order to form such stator magnetic field as is mentioned above a number of coils are required, and, generally speaking, it is desired to arrange a separate coil to each of the stator electrodes. This makes it impossible to design the device to be compact and small-sized.
Accordingly, in the case of the recent small-sized floppy disk drive, particularly the device driving the micro-floppy disk, the conventional stepping motor mentioned above cannot meet the requirement for a compact and small-sized floppy disk drive.
In order to improve the above mentioned stepping motor such a stepping motor is offered that a pair of stator coils are provided and the magnetic flux is distributed and supplied from those coils to a plurality of stator coils to design a small-sized devie. This system, however, cannot produce equal magnetic attraction in the circumferential direction but produces biased magnetic attraction to the rotor, depending on the combination of excitation phases in the stator coils, and the motor shaft is provided with the unignorable inclination.